It is well established that Borrelia burgdorferi, the etiologic agent of Lyme disease, modulates gene expression during infection as it moves between an arthropod vector and mammalian hosts. Several genes required for the establishment of mammalian infection are known with the prototypical gene being ospC. The lipoprotein OspC is absolutely required for mammalian infectivity, and although a putative ligand-binding domain is essential for infectivity, the exact function of OspC is not known. Subsequent studies indicated that ospC is coordinately regulated via a response regulator (Rrp2) that, together with RpoN, drives the expression of RpoS, which then promotes the transcriptional activation of ospC and other infectivity-associated borrelial genes. However, the activation of ospC is transient as it is repressed following infection. In this regard, if ospC expression is made constitutive, the spirochetes are rapidly cleared. Despite this observation, the kinetics of ospC expression, i.e., the amplitude and diminution within a living system over time, is not known. Recently we have used in vivo imaging to detect light emitting (i.e., luciferase [luc] expressing) infectious B. burgdorferi following needle inoculation in mice. The advantage of this approach is that B. burgdorferi can be visualized numerous times in live mice over time to track the infectious process. Given the sensitivity of this technique, an additional potential applicatio might be to assess the expression of targeted genes. To test this premise, we have fused the ospC promoter (PospC) to luc. Our Preliminary Data suggests that ospC is highly expressed early in the infectious process within skin, but is significantly reduced later in the infection, consistent with prior reports indicating that it is down regulated following colonization and dissemination. The utility of this approach will now be expanded to further study the spatial expression or ospC as well as other genes that are coordinately regulated with ospC via RpoS. To this end we propose the following Specific Aims: (1) Characterize the in vivo tissue tropism and temporal production of borrelial ospC utilizing a dual bioluminescence reporter system; and (2) Determine the in vivo expression patterns of genes involved in the Rrp2-RpoN-RpoS regulatory pathway. In the proposed studies, the fate of ospC transcription will be tracked following disseminated infection as well as genes involved in the infectious process to determine if RpoS regulation is highly coordinated or occurs at differential times as B. burgdorferi disseminates. The ability to visualize these regulatory patterns of specific borrelial promoter-luc constructs, focusing on the promoters of genes known to be involved in experimental mouse infection, should provide important insight into the hierarchy and/or temporal expression of these loci to establish and maintain an infectious focus. This exciting approach provides a powerful non-invasive, real time modality to evaluate the activity of a given promoter in a temporal and spatial manner.